1. Field of the Invention
This invention relates to apparatus for fusing toner images to a support by means of heat and pressure and more particularly to fusing apparatus for maintaining uniform fusing quality by varying the fusing pressure as a function of variations in fusing temperature.
2. Description of the Prior Art
In a known type of electrographic reproduction apparatus, a copy of an original is produced by uniformly charging a photoconductive member and exposing it to a light image of the original to form a latent electrostatic image corresponding to the original. The latent image is developed into a visible image through the application of charged toner particles. The toner image is then transferred to a support such as a copy sheet which ultimately becomes the final copy of the original. Typically the toner particles are fused to the copy sheet by advancing the sheet through the nip of a pair of rollers at least one of which is heated. These rollers serve to fuse the toner particles to the copy sheet through the application of a combination of heat and pressure.
In order to increase their life, fuser rollers are typically engaged only during a copy run and disengaged when copy sheets are not being processed. Although the degree of pressure engagement between the fuser rollers may be manually adjustable, once a desired pressure has been selected, the pressure will be maintained constant during each copy run until such time as a new pressure is manually selected. Commonly, the temperature at the surface of the heated roller is maintained within a preselected operating range which is consistant with the selected pressure in order to effect proper fusing of toner images to copy sheets.
It has been found that in long copy runs the amount of heat lost to the copy sheets causes a drop in the amount of heat supplied to the toner images of successive copy sheets resulting in a drop in fusing quality to undesirable levels. Control of fusing temperature alone is not completely satisfactory, since there is an unavoidable delay between the time a change of temperature is sensed and the time the heated roller can be brought to a new temperature.